Purpose: This study simulated regular QA procedures in order to examine relationships between offsets in beam-shaping devices and delivered dose. The goal was to provide insight into the cause of potential QA irregularities, and to investigate whether current QA procedures are adequate to detect equipment misalignments.

Methods: The Monte Carlo package Topas, based on Geant4, was used to simulate scattering nozzle QA procedures. Scoring volumes in the nozzle geometry mimic online treatment verification devices including ion chambers and a multi-layer faraday cup range verifier, as well as weekly and monthly measurements of SOBP depth-dose curves and beam profiles. The study then examined the relationship between geometric perturbations to the Monte Carlo model and changes in scoring outputs. Small movements of equipment including scatterers, ion chambers, and range modulators were considered.

Results: A number of different insights have come from our investigation. Of particular note, we found that the measurement of beam profile skewness in an upsteam ion chamber is inadequate to use for beam steering to correct for asymmetries in the delivered dose. Skewness alone cannot distinguish between an asymmetric beam due to scatterer misalignment and a simple offset in the ion chamber location. For misalignments in the range modulator, typically no significant change is seen in online measurements before a change occurs in the output factor or modulation width, reinforcing the need for regular measurement of standard beams to verify these quantities. Comparing simulation and measurement can give hints at actual misalignments of nozzle hardware.

Conclusion: Monte Carlo techniques can give useful insight into potential QA pitfalls. Through simulation we found that while ion chamber skewness can detect beamline asymmetries, it is inadequate to use for beam steering to correct a tilted beam profile. Standard online measurements have difficulty detecting longitudinal dose abnormalities compared to frequent direct measurement.